Heretofore, thermoplastic elastomers have been widely used as materials for the manufacture of automotive parts such as bumper. The thermoplastic elastomers have characteristics of both thermoplasticity and elasticity, and are capable of being formed by injection or extrusion molding into molded articles which are excellent in heat resistance, tensile properties, weatherability, flexibility and elasticity.
For example, Japanese Patent Publication No. 53-34210 discloses thermoplastic elastomers prepared by partially curing a blend of from 60 to 80 parts by weight of a monoolefin copolymer rubber and from 40 to 20 parts by weight of a polyolefin plastics under dynamical conditions. Japanese Patent Publication No. 53-21021 discloses thermoplastic elastomers comprising (a) a partially crosslinked copolymer rubber comprising a ethylene/propylene/non-conjugated polyene copolymer rubber having a gel content of from 30 to 90% by weight and (b) a polyolefin resin. Further, Japanese Patent Publication No. 55-18448 discloses thermoplastic elastomers prepared by partially or fully curing a blend of an ethylene/propylene copolymer rubber and a polyolefin resin under dynamical conditions.
Japanese Patent Laid-open Publication No. 58-187412 discloses a crosslinked block copolymer derived from an olefinic copolymer comprising from 50 to 70 parts by weight of one or more blocks [A] selected from homopolymer blocks of propylene and binary random copolymer blocks of propylene and ethylene of a C4-12 .alpha.-olefin and having a propylene content of from 100 to 60% by weight, and from 30 to 50 parts by weight of one or more blocks [B] selected from binary random copolymer blocks of ethylene and propylene and having an ethylene content of from 30 to 85% by weight, said crosslinked block copolymer having a specified content of hot xylene insoluble components and a specified fluidity.
Japanese Patent Laid-open Publications Nos. 63-165414, 63-165415 and 63-415416, and U.S. Pat. No. 4,454,306 disclose a process for preparation of a crosslinked olefinic block copolymer which comprises melt kneading an olefinic block copolymer comprising one or more homopolymer blocks of propylene [A], one or more first binary random copolymer blocks of propylene and ethylene [B]and one or more second binary random copolymer blocks of propylene and ethylene [C] and prepared using a specific Tiegler's catalyst, together with an organic peroxide, a divinyl compound and an antioxidant, at a temperature of not higher than 230 .degree. C.
Japanese Patent Laid-open Publication No. 48-21731 discloses a method for improving a processability of block copolymers wherein a block copolymer comprising from 3 to 30% by weight of copolymer segments primarily comprised of ethylene and containing up to 70% by weight of at least one other .alpha.-olefin and from 97 to 70% by weight of polymer segments primarily comprised of propylene is admixed with an organic peroxide and the admixture is heat treated at a temperature of from 180.degree. to 270.degree. C.
We have carried out extensive researches on the preparation of thermoplastic elastomers by direct dynamic heat treatment of polymer particles, and found that if polymer particles having a specific morphology are so treated, there can be economically prepared thermoplastic elastomers which are very smooth, excellent in strength and elasticity even though they have a reduced rubber content, and capable of being molded into articles having good appearance, in particular, good appearance after painted. The invention is based on the findings.
Thus, an object of the invention is to provide processes for the preparation of thermoplastic elastomers which have excellent elasticity even with a reduced rubber content and excellent strength, and are capable of being molded to articles which are very uniform, and excellent in strength properties such as tensile strength, heat resistance, weatherability, flexibility, elasticity, surface smoothness, properties of being painted and economy.